doc: improve buf.indexOf() documentation style

* improve usage of "coerce" in buffer.md
* reduce informal tone in buffer.md
* wrap line at 80 characters in buffer.md
* wrap value in backtick for consistency

PR-URL: nodejs#19861
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>

doc/api/buffer.md

@@ -1240,10 +1240,9 @@ changes:
 
 * `value` {string|Buffer|Uint8Array|integer} What to search for.
 * `byteOffset` {integer} Where to begin searching in `buf`. **Default:** `0`.
-* `encoding` {string} If `value` is a string, this is the encoding used to
-  determine the binary representation of the string that will be searched for in
-  `buf`. **Default:** `'utf8'`.
-* Returns: {integer} The index of the first occurrence of `value` in `buf`, or
+* `encoding` {string} The encoding of `value` if `value` is a string.
+  **Default:** `'utf8'`.
+* Returns: {integer} The index of the first occurrence of `value` in `buf` or
   `-1` if `buf` does not contain `value`.
 
 If `value` is: